The present invention relates to nuclear fuel assemblies, and more particularly, to applying absorptive coatings on fuel assembly components.
For many years it has been common practice to include in the core of light water nuclear reactors, neutron-absorbing material distributed non-homogeneously among the fuel assemblies, to shape the distribution of power generated in the core. More particularly, so-called "burnable poison" is typically in the form of dedicated rods distributed throughout some or all of the fuel assemblies, containing a quantity of boron, for example in the form of B.sub.4 C, which is highly effective in absorbing neutrons when initially placed in the core, but which diminishes in absorptive effect during one or more burnup cycles such that the effect eventually becomes essentially zero.
One disadvantage of such conventional burnable poison rods, is that each burnable poison rod replaces a nuclear fuel rod in the assembly, thereby diminishing the total power and total energy that can be extracted from the core during a particular burnup cycle. This disadvantage is more than counter-balanced by the effectiveness of the burnable poison rods in the localized shaping of the power so as to avoid high peak to average power ratios in particular fuel rods. U.S. Pat. No. 3,799,839 provides a general description of the use of burnable poisons in nuclear fuel assemblies. The power shaping permits the core to operate at its gross rated power early in the burnup cycle, but with some sacrifice as to total burnup cycle energy extraction.
In recognition of this significant disadvantage, efforts have been made to incorporate burnable poison material within the fuel rod itself, thereby avoiding the placement of dedicated burnable poison rods in the fuel assembly. U.S. Pat. No. 4,541,984 describes a method for providing burnable poison, getter, and lubricant coatings on the inside of nuclear fuel rods. U.S. Pat. No. 4,824,634 describes the application of a zirconium diboride in natural or enriched form with acrylic polymer binder on the inside of fuel rods. U.S. Pat. No. 4,880,597 describes nickel-thallium-boron electroless plating on the inside of fuel rods. U.S. Pat. No. 4,990,303 discloses enriched boron-10 and glass deposited on the inside of fuel rods using the sol-gel process. It is well known that, in addition to boron, gadolinium and erbium also have properties that are desirable for use as burnable poison material.
It can thus be appreciated that significant effort has been dedicated to developing and perfecting techniques for applying burnable poison material to the inside surface of nuclear fuel rod cladding tubes. These tubes, which are typically on the order of 0.375 inch ID, present formidable difficulties due not only to the small diameter, but also to the typical length of at least about 12 ft. Providing a uniform coating on the inside surface over substantially the full length of such tubes, without wasting expensive coating material or producing areas of excessive thickness which would interfere with the stacking of fuel rod pellets within the cladding, has presented practical difficulties as well as formidable costs.